THE EGS5 CODE SYSTEM

1.2.2 EGS1
About this time Nelson became interested in being able to use Ford’s version of the code and offered
to help support its further development. One of Ford’s objectives was to make the preprocessor
code produce data for the shower code in a form that was directly usable by the shower code with
a minimum of input required by the user. In SHOWER3/PREPRO and in SHOWER4/SHINP,
whenever it was desired to create showers in a new medium, it was necessary to look-up the photon
cross sections in the literature and keypunch them for the preprocessing code to use. Subsequent to
this it was necessary to select from several fits produced by the preprocessing code and to include
this new information, consisting of many data cards, with other data used by the shower program.
Ford rewrote the preprocessor to automatically produce all of the data needed by the shower code
in a readily acceptable form and, with the assistance of Nelson, obtained photon cross sections for
elements 1 to 100 from Storm and Israel[167] on magnetic tape. Ford also separated the shower
code’s material-input from its control-input. For flexibility and ease of use, the NAMELIST read
facility of FORTRAN-IV was utilized for reading-in control data in both the preprocessor and the
shower codes. The resultant shower code was re-named EGS (Electron-Gamma-Shower) and its
companion code was called PEGS (Preprocessor for EGS). This version, written completely in the
FORTRAN-IV language, is referred to as Version 1 of the EGS Code System (or more simply EGS1
and PEGS1).
The sampling routines were tested using the internal test-procedure facility of EGS1 and, with
the exception of the bremsstrahlung process, were found to be operating very nicely. In the bremsstrahlung
case a ripple, amounting to only 5% but still noticeable, was observed when the sampled
data were compared with the theoretical secondary distribution. This effect went away upon selection
of another random number generator, and it was concluded that correlations in the original
number generator were the cause. EGS1 was then tested against various experiments in the literature
and with other Monte Carlo results that were then available and the authors found reasonable
good agreement in all cases.
1.2.3 EGS2
By the fall of 1974 the Hofstadter group had obtained some hexagonal modular NaI detectors
and the discovery of the J/ψ particle[12, 13] in November, 1974 opened up an exciting area of
high-energy gamma-ray spectroscopy for which the modularized NaI detectors were ideally suited.
EGS1, however, could not be readily used to simulate showers in complex geometries such as those
presented by modular stacks of NaI. A good example of this was the Crystal Ball detector for which
EGS1, under the direction of E. Bloom at SLAC, was modified to handle the particular geometry
in question. Furthermore, Nelson had received a large number of requests from the growing list
of EGS users, both at SLAC and elsewhere in the high-energy physics community, to improve
further EGS1 so that complex geometries could be realized in the near future. Thus it was decided
that EGS1, which was a one-region, one-medium code, should be generalized in order to handle
many-region, many-media, complex, three-dimensional geometries.
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